Method for purifying, cooling and separating a gaseous mixture and associated apparatus

ABSTRACT

The invention relates to a method for cooling, purifying and separating a gaseous mixture containing at least one impurity, in which the gaseous mixture is cooled to a temperature no higher than the temperature at which the at least one impurity solidifies in a heat exchanger having cooling passages, the cooling passages being at least partially covered with a coating and/or physically treated and/or chemically treated, the coating and/or the treatment serving to limit or even prevent the solidified impurity from forming and/or adhering to a surface of the passages; at least one portion of the solidified impurity exiting the cooling passages of the heat exchanger is collected; and the gaseous mixture is withdrawn from the heat exchanger.

The present invention relates to a method for purifying, cooling and separating a gas mixture and to an apparatus for purifying and cooling a gas mixture. When a gas mixture must be cooled to a temperature below the liquefaction temperature, or even the solidification temperature of one of the gaseous components that it contains, this poses particular problems. If the heat exchanger used for cooling the gas mixture is, for example, a plate-fin exchanger comprising passages in which the gas mixture is cooled, these passages risk being obstructed by the formation of solids on the walls of the passages.

This problem is particularly well known to air separation specialists since conventionally the air to be distilled was cooled in heat exchangers and the water and carbon dioxide contained in the air were deposited on the walls of the passages of a first exchanger. Before the blocking of these passages, the air was sent to another heat exchanger in order to be cooled while the first heat exchanger was reheated by passing nitrogen therethrough in order to melt then vaporize the water and remove the carbon dioxide.

These systems were abandoned several decades ago in order to be replaced by a purification upstream of the heat exchanger. In this strategy that has become common, the air is dried and decarbonated by an adsorption process in front-end purification and then cooled.

One object of the present invention is to propose an alternative to the conventional strategies for the purification and cooling, or even liquefaction, of a gas mixture.

One object of the present invention is to propose an alternative to the conventional purification strategies for air separation units and for other separation and/or liquefaction units, operating at low temperature. Among these liquefaction units, mention may be made of air liquefaction units (for example for storing energy, units for liquefaction of a gas produced by separating air, for example nitrogen and units for liquefaction of natural gas. Other examples of separation units comprise for example the units for separating a mixture of carbon dioxide containing at least 30% carbon dioxide and also hydrogen and/or methane and/or oxygen and/or carbon monoxide at subambient temperature. The separation units may also be units for the cryogenic separation of mixtures of hydrogen and/or carbon monoxide and/or nitrogen and/or methane, the mixture containing at least 10 mol % of at least one of these components.

The study of new surface treatment technologies makes it possible to envisage drying and/or decarbonating the air directly in the main exchanger in such a way that the water condensed then frozen and/or the frozen CO₂ have a reduced adhesion or even so that they no longer adhere to the walls of this exchanger and therefore so that the clogging increases until the passages on the air side of the exchanger are blocked.

It is known to treat surfaces or cover them with a coating in fields very different from the low-temperature separation of a gas mixture, in particular to reduce, or even prevent, the adhesion of ice or frost on metal surfaces exposed to atmospheric elements, such as aircraft, wind turbines and pylons. These surfaces may also, in certain cases, reduce the amount of ice formed.

A large number of surface treatments make it possible to reduce the adhesion of ice. These are treatments referred to as “passive anti-ice treatments” which in general are based on silicone or fluorocarbon polymers (non-exhaustive list) as described in US-A-2013/0305748.

For example, polytetrafluoroethylene, PTFE, also known under the name Teflon®, allows a weak adhesion of ice owing to a low surface tension as described in M. G. Ferrick, N. D. Mulherin, B. A. Coutermarsh, G. D. Durell, L. A. Curtis, T. L. St. Clair, E. S. Weiser, R. J. Cano, T. M. Smith, C. G. Stevenson and E. C. Martinez, Journal of Adhesion Science and Technology, 26 (2012) 473. The polysiloxane-based NUSIL® R2180 coating also makes it possible to significantly reduce the adhesion of ice.

Similar results have been obtained using another coating based on (DLC or examples of coatings that make it possible to limit the adhesion of ice to aluminum alloys are reported by Menini et al., Cold Regions Science and Technology, 65 (2011) 65.

In most cases, these treatments increase the hydrophobicity of the surfaces, which makes it possible to increase the contact angle of water on these surfaces and therefore to reduce the interactions between water and the surface. Thus, the surfaces that make it possible to reduce the adhesion of ice are in general hydrophobic or superhydrophobic as described in L. Foroughi Mobarakeh, R. Jafari and M. Farzaneh, Applied Surface Science, 284 (2013) 459.

It is also possible to reduce the adhesion of ice using heterogeneous surfaces comprising both hydrophobic and hydrophilic zones as described in WO-A-05075112. In this case, the treatment makes it possible to properly control the water crystallization zones, which facilitates the elimination of the ice with the aid of a flushing of gas or liquid.

Another type of surface makes it possible to reduce the adhesion of the ice, these are lubricated surfaces. The surface is impregnated with a lubricant which may be based on fluorocarbons such as Krytox® or on silicone oils as described in WO-A-2012/100100 and US-A-2006/0281861.

With such surfaces, the ice is in contact with the lubricant, i.e. a liquid phase, thus the adhesion forces are very weak. The lubricant also has another advantage, it makes it possible to improve the erosion resistance of the surfaces.

In the example cited so far, the surface coatings make it possible to reduce the adhesion of the ice. Another type of surface that may prove advantageous within the context of this invention are the coatings referred to as “active anti-ice coatings” that make it possible to slow down the formation of the ice.

It is known that it is possible to lower the ice formation temperature using salt or glycol-type compounds. It turns out that a similar phenomenon may take place when a polymer-type compound, in general a hygroscopic polymer, is grafted to a surface.

This type of coating may lower the ice formation temperature and reduce the amount of ice formed. The best known coatings in this regard are of glycol type (US-A-2010/0086789) but anti-frost coatings inspired by the structure of proteins have a similar effect as described in L. Makkonen, Journal of Adhesion Science and Technology, 26 (2012) 413.

Finally, certain treatments make it possible to combine two effects: modifying the type of frost and reducing its adhesion as described by J. Chen, R. Dou, D. Cui, Q. Zhang, Y. Zhang, F. Xu, X. Zhou, J. Wang, Y. Song and L. Jiang, ACS Applied Materials and Interfaces, 5 (2013) 4026. This is the case for microstructured surfaces comprising a hydroscopic polymer matrix (based on polyacrylic acid for example). These surfaces make it possible to easily eliminate the ice formed and have the advantage of using water as lubricant, thus the surface is self-supplied with lubricant with the aid of the humidity contained in the air.

It is also known to limit, or even prevent the formation of frost by heating, for example with the Joule effect or pneumatic pulsation systems.

There are also techniques with a gas stream with a sufficient flow or an input of mechanical and/or electrical energy, which may be combined with a coating and/or a treatment in order to easily detach the impurities, the adhesion of which has been reduced.

A person skilled in the art who is a specialist in low-temperature purification and cooling or separation or liquefaction processes is not abreast with developments relating to the surface treatments and the coatings used for limiting, or even preventing the formation and/or the adhesion of ice on a surface. A portion of the present invention results from the realization that these techniques may be applied to the field of cooling and purification, for example used upstream of a separation or a liquefaction, for example a distillation, at low temperature or another process operating at low temperature.

GB-A-917286 describes a low-temperature separation process in which a gas containing carbon dioxide is cooled by two exchangers in rotation, each allowing a heat exchange between only two fluids and each comprising a zone designed to avoid the deposition of carbon dioxide.

In this case, as the passages intended to cool the gas to be separated during a first period are also used to reheat a separated gas during a second period, it is necessary to provide the same number of passages and the same type of fins in the exchanger in order to obtain a symmetrical operation during the two operating periods.

The heat exchanger from GB-A-917286 inevitably consists of at least two exchange bodies and cannot be a monoblock body.

A low-temperature separation takes place at at most 0° C., or even at at least −50° C., or even at at least −100° C., depending on the gas mixture to be separated.

The “hot end” is the portion of the heat exchanger that is found operating at a maximum mean temperature. The “cold end” is that portion of the heat exchanger that is found operating at a minimum mean temperature.

A heat exchanger is a single exchange body or a plurality of exchange bodies, capable of carrying out a heat exchange.

The gas mixture to be cooled enters at the hot end of the exchanger and leaves therefrom, generally at the cold end.

Generally, a heat exchanger is mounted so that its hot end is found toward the top and its cold end toward the bottom. In certain cases, as described later on, the present invention may necessitate placing the hot end at the bottom and the cold end at the top.

The heat exchanger is generally placed inside a thermally insulated cold box. Other elements of a separation apparatus may also be found in the cold box, for example a distillation column.

The conclusions regarding the purification of a gas containing water and CO₂ are that the purification of the gas by condensation/solidification is more energetically efficient than adsorption. It also makes it possible to eliminate or reduce in size the apparatus used according to the prior art, for example the apparatus for purification particular advantages since being colder at the outlet of the exchanger, the CO₂, and especially a portion of the secondary impurities, are stopped even better, which makes it possible to simplify, or even eliminate the downstream purifications and/or to simplify the design and/or the operation of certain downstream equipment (for example the vaporizers).

According to one subject of the invention, a process is provided for cooling, purifying and separating a gas mixture containing at least one impurity, wherein the gas mixture containing at least one impurity is cooled to a temperature below or equal to that at which the at least one impurity solidifies in a heat exchanger comprising at least one exchange body having cooling passages designed to reduce the adhesion of the solidified impurity on a surface of the passages, at least one portion of the solidified impurity leaving the cooling passages of the heat exchanger and/or at an intermediate level of the heat exchanger is collected and the gas mixture, optionally at least partially liquefied, is drawn off from the heat exchanger, preferably at the cold end, the gas mixture is optionally cooled again and the gas mixture is sent to a system of columns in order to be separated by distillation at low temperature, or even cryogenic temperature, in order to produce two fluids, each enriched in one component of the gas mixture, characterized in that the cooling passages are at least partially covered by a coating and/or physically treated and/or chemically treated, the coating and/or the treatment serving to limit, or even prevent, the formation and/or the adhesion of the solidified impurity on a surface of the passages, and in that

i) during substantially the entire time that the separation by distillation is carried out, the gas mixture is cooled in each exchange body having cooling passages designed to reduce the adhesion of the solidified impurity on a surface of the passages and/or

ii) the two fluids, each enriched in one component of the mixture, are reheated in each exchange body having cooling passages designed to reduce the adhesion of the solidified impurity on a surface of the passages.

According to other optional aspects:

-   -   the gas mixture drawn off contains at least one portion of the         solidified impurity,     -   the gas mixture comprises at least one component, preferably a         major component, that is not solidified and optionally that is         not liquefied in the heat exchanger,     -   the gas mixture comprises at least two components, preferably         major components, that are not solidified and optionally that         are not liquefied in the heat exchanger,     -   an impurity is a component that represents no more than 10 mol %         or 5 mol % or even 1 mol %, or even 0.1 mol %, or even 0.01 mol         % of the gas mixture,     -   the gas mixture is purified to remove at least one fraction of         the at least one impurity upstream of the heat exchanger, this         fraction representing between 20% and 95% of the impurity         contained in the gas mixture upstream of the process,     -   the gas mixture is not purified to remove at least one fraction         of the at least one impurity upstream of the heat exchanger,     -   at least one portion of the solidified impurity is collected         downstream of the heat exchanger, by means of a phase separator         and/or an endless screw,     -   the cooling passages are at least partially modified physically,         the treatment serving to limit, or even prevent, the formation         and/or the adhesion of the solidified impurity on a surface of         the passages,     -   the cooled gas mixture is treated downstream of the heat         exchanger and/or at at least one intermediate level of the heat         exchanger in order to eliminate the impurity in gaseous and/or         liquid and/or solid form,     -   the cooled gas mixture is treated only downstream of the heat         exchanger and not at at least one intermediate level of the heat         exchanger in order to eliminate the impurity in gaseous and/or         liquid and/or solid form,     -   the cooled gas mixture is treated only at at least one         intermediate level of the heat exchanger and not downstream of         the heat exchanger in order to eliminate the impurity in gaseous         and/or liquid and/or solid form,     -   the gas mixture is cooled in the heat exchanger firstly to a         temperature below or equal to the liquefaction temperature of         the at least one impurity but above its solidification         temperature, the gas mixture is taken out of the exchanger in         order to eliminate a portion of the impurity in liquid form and         the gas mixture containing some impurity is sent back to the         heat exchanger in order to cool it to the solidification         temperature of this impurity,     -   at least 20%, at least 50%, or even at least 70%, or even at         least 90% of the impurity present at the inlet of the heat         exchanger, referred to as the hot end, is eliminated by         collecting it at at least one intermediate point of the heat         exchanger after cooling, in solid and/or liquid form,     -   at most 80%, at most 50%, or even at most 30%, or even at most         10% of the impurity present at the inlet of the heat exchanger,         referred to as the hot end, is eliminated by collecting it at at         least one intermediate point of the heat exchanger after         cooling, in solid and/or liquid form,     -   at least 50%, or even at least 70%, or even at least 90%, or         even at least 99%, or even at least 99.9%, or even at least         99.99% of the impurity present at the inlet of the heat         exchanger, referred to as the hot end, is eliminated by         collecting it downstream of the heat exchanger after cooling up         to the outlet of the exchanger at the cold end,     -   at most 50%, or even at most 30%, or even at most 10%, or even         at most 1%, or even at most 0.1%, or even at most 0.01% of the         impurity present at the inlet of the heat exchanger, referred to         as the hot end, is eliminated by collecting it downstream of the         heat exchanger after cooling up to the outlet of the exchanger         at the cold end,     -   at least 20%, at least 50%, or even at least 70%, or even at         least 90%, or even at least 99% of the impurity present,         upstream of the inlet of the heat exchanger, referred to as the         hot end, is eliminated,     -   at most 80%, at most 50%, or even at most 30%, or even at most         10%, or even at most 1% of the impurity present, upstream of the         inlet of the heat exchanger, referred to as the hot end, is         eliminated,     -   the hot end of the heat exchanger is placed at a higher level         than that of the cold end,     -   the hot end of the heat exchanger is placed at a lower level         than that of the cold end or than that of an intermediate level         of the exchanger in the case of an inverted U-shaped exchanger         and at least 50% of the impurity, for example water, present in         the gas mixture to be cooled is eliminated at the inlet of the         heat exchanger, referred to as the hot end, by collecting it in         solid or liquid form at the hot end of the exchanger where it         drops by gravity after cooling in the heat exchanger (since its         adhesion to the walls is limited),     -   the hot end of the heat exchanger is placed at the same level as         that of the cold end in the case of an inverted U-shaped         exchanger,     -   the gas mixture is air or a mixture having, as main components,         hydrogen and/or carbon monoxide and/or methane and the at least         one impurity is water and/or carbon dioxide, or a mixture         having, as main component, carbon dioxide and optionally         hydrogen and/or carbon monoxide and/or methane and/or oxygen         and/or nitrogen and/or argon and the at least one impurity is         water,     -   at least one surface of the cooling passages has been treated to         make it rougher and/or to lubricate it and/or to make it         hydrophilic and/or hydrophobic and/or hydroscopic and/or         hygroscopic in order to limit, or even prevent, the formation         and/or the adhesion of solidified impurities, for example ice,     -   the heat exchanger may comprise passages, at least one section         of which has a treatment and/or a coating and/or a geometry         and/or, in the case of a plate-fin exchanger, a type of fin that         differs from that of another section that has to operate at a         lower temperature range,     -   the heat exchanger may comprise passages, at least one section         of which has a treatment and/or a coating and/or a geometry         and/or, in the case of a plate-fin exchanger, a type of fin that         differs from that of another section that is found downstream of         an intermediate point for drawing off solidified impurities,     -   the gas mixture is purified and cooled by a process as described         above, optionally cooled again and sent to a system of columns         in order to be separated by distillation at low temperature, or         even cryogenic temperature, in order to produce at least one         fluid enriched in one component of the gas mixture,     -   the fluid enriched in one component of the gas mixture is         reheated in the heat exchanger in the reheating passages,     -   the heat exchanger comprises at least one passage for reheating         a fluid, the at least one reheating passage not having been         treated or coated in order to limit, or even prevent, the         formation and/or the adhesion of solidified impurities, for         example ice,     -   at least one portion of the solidified impurity leaving the         cooling passages of the heat exchanger is sent back to the heat         exchanger to be reheated,     -   at least one portion of the solidified impurity is mixed with         another gas before being reheated in the heat exchanger,     -   the at least one reheating passage to which the solidified         impurities are sent has been treated or coated in order to         limit, or even prevent, the formation and/or the adhesion of         solidified impurities, for example ice,     -   at least one portion of the solidified impurity leaving the         cooling passages of the heat exchanger and/or at an intermediate         level of the heat exchanger is collected and the gas mixture         finds itself liquefied or is liquefied by a subsequent step         downstream of the exchanger and/or is separated at a subambient         temperature downstream of the exchanger, optionally after         elimination, downstream of the exchanger, of remaining         impurities that would be solidified at this subambient         temperature,     -   frigories are supplied to the gas mixture which is cooled at at         least one intermediate point of the heat exchanger,     -   frigories are supplied to the gas mixture which is cooled at at         least one intermediate point of the heat exchanger, preferably         downstream or upstream of a point for drawing off at least one         portion of the solidified or liquefied impurity at an         intermediate level of the heat exchanger,     -   frigories are supplied to the gas mixture which is at a         temperature between −5° C. and 5° C.,     -   frigories are supplied to the gas mixture which is at a         temperature between −20° C. and −30° C.,     -   frigories are supplied to the gas mixture by taking at least one         portion of the gas mixture out of the heat exchanger and by         cooling it,     -   frigories are supplied to the gas mixture by means of a         refrigerant fluid sent to an intermediate level of the heat         exchanger,     -   the gas mixture is cooled in the heat exchanger continuously,     -   the gas mixture is cooled in the heat exchanger intermittently,     -   the heat exchanger cools the gas mixture up to the cold end to a         temperature below 0° C., or even below −50° C., or even below         −100° C.,     -   the gas mixture is at atmospheric pressure or a pressure above         atmospheric pressure,     -   the heat exchanger comprises only a single exchange body and is         a monoblock exchanger,     -   the heat exchanger comprises at least two exchange bodies,     -   the gas mixture is cooled in cooling passages, the number of         which is not equal to the number of reheating passages connected         to the means for transporting the first gas,     -   the gas mixture is cooled in cooling passages, the number of         which is not equal to the number of reheating passages connected         to the means for transporting the second gas.

Each of the above features may be combined with each of the others above except in the case of an obvious incompatibility.

If the passages of the heat exchanger are treated to limit the deposition of impurities but not to prevent it completely, it will be necessary to remove the solids formed in the passages, for example by heating and/or by passage of the gas mixture at a sufficient flow rate (its nominal flow rate or a higher flow rate) and/or at high pressure relative to the flow rate and/or pressure that are used during the cooling or by mechanical means, for example by variation of the gas mixture flow rate or pulses of the gas mixture flow rate, or else vibrations applied directly to the exchanger.

According to one subject of the invention, an apparatus is provided for cooling and purifying a gas mixture containing at least one impurity comprising a heat exchanger having passages for cooling the gas mixture and passages for reheating a gas, means for sending the gas mixture containing at least one impurity to be cooled in the heat exchanger to a temperature below or equal to that at which the at least one impurity solidifies and means for drawing off the, optionally at least partially liquefied, gas mixture from the heat exchanger, preferably at the cold end and means for collecting at least one portion of the solidified impurity leaving the cooling passages of the heat exchanger and/or at an intermediate level of the heat exchanger and means for taking the gas mixture of the at least one impurity out of the heat exchanger, characterized in that the cooling passages are at least partially covered by a coating and/or physically treated and/or chemically treated, the coating and/or the treatment serving to limit, or even prevent, the formation and/or the adhesion of the solidified impurity on a surface of the passages and in that the apparatus comprises a single heat exchanger connected to means for collecting at least one portion of the solidified impurity, this heat exchanger being a monoblock heat exchanger.

According to other optional aspects:

-   -   the apparatus comprises reheating passages for a first gas and         reheating passages for a second gas,     -   the apparatus comprises means for purifying the gas mixture         upstream of the heat exchanger in order to remove at least one         fraction of the at least one impurity,     -   the means for collecting at least one portion of the solidified         impurity downstream of the heat exchanger are constituted by a         phase separator and/or an endless screw,     -   the heat exchanger is constituted by at least one plate-fin         exchanger,     -   the heat exchanger is constituted by at least two plate-fin         exchangers made of aluminum or made of copper or made of         titanium,     -   the heat exchanger is constituted by at least one coil         exchanger,     -   the heat exchanger is constituted by at least one shell and tube         exchanger,     -   the apparatus comprises means for treating the cooled gas         mixture downstream of the heat exchanger and/or at at least one         intermediate level of the heat exchanger in order to eliminate         the impurity in gaseous and/or liquid and/or solid form,     -   the hot end of the heat exchanger is placed at a higher level         than that of the cold end,     -   the hot end of the heat exchanger is placed at a lower level         than that of the cold end or than that of an intermediate level         of the exchanger in the case of an inverted U-shaped exchanger         and at least 50% of the impurity, for example water, present in         the gas mixture to be cooled is eliminated at the inlet of the         heat exchanger, referred to as the hot end, by collecting it in         solid or liquid form at the hot end of the exchanger where it         drops by gravity after cooling in the heat exchanger,     -   at least one surface of the cooling passages has been treated to         make it rougher and/or to lubricate it and/or to have a surface         that is hydrophobic and/or superhydrophobic and/or that has         hydrophobic and hydrophilic, and/or hygroscopic zones, in order         to limit, or even prevent, the formation and/or the adhesion of         solidified impurities, for example ice,     -   the surface of a passage may or may not be impregnated with a         lubricant,     -   the exchanger comprises at least one passage for reheating a         fluid and at least one surface of the at least one reheating         passage has been treated to make it rougher and/or to lubricate         it and/or to make it hydrophilic and/or hydrophobic and/or         hydroscopic and/or hygroscopic in order to limit, or even         prevent, the formation and/or the adhesion of solidified         impurities, for example ice.

According to another aspect of the invention, an apparatus is provided for separation by distillation at low temperature, or even cryogenic temperature, comprising a cooling and purification apparatus as described above and also a system of columns and means for sending the gas mixture purified and cooled by the cooling and purification apparatus to the system of columns.

The separation apparatus may not comprise means for cooling the gas mixture downstream of the cooling and purification apparatus.

The apparatus may comprise means for sending a fluid enriched in one component of the gas mixture to be reheated in the heat exchanger in reheating passages.

According to other optional features:

-   -   the heat exchanger comprises at least one passage for reheating         a fluid, the at least one reheating passage not having been         treated or coated in order to limit, or even prevent, the         formation and/or the adhesion of solidified impurities, for         example ice,     -   the apparatus comprises means for sending at least one portion         of the solidified impurity leaving the cooling passages of the         heat exchanger to the heat exchanger in order to be reheated,     -   means for mixing at least one portion of the solidified impurity         with another gas before being reheated in the heat exchanger,     -   the at least one reheating passage to which the solidified         impurities are sent has been treated or coated in order to         limit, or even prevent, the formation and/or the adhesion of         solidified impurities, for example ice.

The separation and purification apparatus may comprise means for supplying frigories to the gas mixture which is cooled at at least one intermediate point of the heat exchanger.

The separation and purification apparatus may comprise means for supplying frigories to the gas mixture which is cooled at at least one intermediate point of the heat exchanger, preferably downstream and/or upstream of a point for drawing off at least one portion of the solidified or liquefied impurity at an intermediate level of the heat exchanger.

The separation and purification apparatus may comprise means for supplying frigories to the gas mixture by taking at least one portion of the gas mixture out of the heat exchanger.

The separation and purification apparatus may comprise means for supplying frigories to the gas mixture by means of a refrigerant fluid sent to an intermediate level of the heat exchanger.

In all the strategies for separation by liquefaction and condensation of the impurities, the issue is to compensate for the phase change enthalpies of the various constituents by input of energy (here refrigeration make-up) via devices external to the main exchanger (example: heat pump, refrigerating unit).

If no refrigeration make-up is made, the exchange graph is moved apart at the cold end as is seen in FIG. 11.

In the curve from FIG. 11, the curvature caused by the condensation then the solidification of water (moist air at 1.4 bara) is observed. FIG. 12 is a graph that is “compensated for” by sufficient refrigeration make-ups.

The case of FIG. 12 is a simulation of the condensation then the solidification of water combined with the solidification of CO₂.

The invention will be described in greater detail by referring to FIGS. 1 to 10 which schematically represent processes according to the invention.

Detailed here are process diagrams to which the concept of the invention could be applied. They are diagrams of a single-column, low-pressure air separation unit. They could be transposed to other separation and/or liquefaction processes, such as cryogenic separation processes for the H₂/CO mixture as explained above.

Represented in FIG. 1 is a process for separating air by cryogenic distillation using a brazed aluminum plate-fin heat exchanger 3 and a single distillation column 27. This process enables the production of an oxygen-enriched liquid 43, an oxygen-enriched gas 45 and a nitrogen-enriched gas 47. The use of a brazed aluminum plate-fin heat exchanger is not essential. This exchanger may use other technologies and may for example be a coil exchanger or a shell and tube exchanger.

The air to be separated 1 contains water and carbon dioxide, which must be purified upstream of the distillation. After filtering through a filter F and compression in a compressor C, the compressed air 1 enters the heat exchanger 3 constituted by a single exchange body and referred to as an “exchange line” without passing through beds of adsorbents conventionally present in an air separation apparatus. It can be envisaged to eliminate a portion of the water contained in the air by separating the water that is condensed, during the compression of the air followed by a cooling step. However, at least 20% of the water present in the ambient air will be removed by passing through the exchanger. The extraction of the water on the one hand then the remainder of the water and the CO₂ on the other hand are carried out at two different locations in the exchange line 3. A large portion of the water is removed in liquid form (around 75% of the water present in the air 1 on arriving in the exchanger 3, after compression followed by a cooling step) at a temperature close to 0° C.: the air 5 at this location is drawn off by separating the air and the water 5B in a phase separator 2 then the dried air 5A is reinjected in order to finish the cooling thereof and to carry out the same separation at its outlet from the exchange line 3 with the remainder of the water and the CO₂ this time, the two being solid. To compensate for the latent heat of liquefaction and of condensation of the impurities, two refrigeration make-ups are necessary via two heat pumps, for example at 0° C. and at −25° C.

Thus air 7 drawn off at an intermediate level of the exchange line 3 is cooled by means of a first heat pump 4 and the cooled air is sent back to the exchange line 3.

Next air 11 drawn off at a colder intermediate level of the exchange line 3 is cooled by a second heat pump 6 supplied by a fluid 13. The cooled air 11A is sent back to the exchange line.

The air already purified of water and cooled in two steps 15 contains ice and solid carbon dioxide and is sent to a phase separator 17 and the ice and the solid carbon dioxide 19 are removed.

The walls of the cooling passages are treated in order to limit, even prevent, the formation and/or the adhesion of ice and of carbon dioxide to the surfaces, at least in the regions where the temperature of the passage is anticipated to be below the solidification temperature of the water and/or of the carbon dioxide.

This treatment may be a physical treatment of the surface or the installation of water and solid carbon dioxide remain in the air and pass through the exchange line to the cold end before being collected in the second phase separator 17.

A portion of the secondary impurities of the air (in particular propane, acetylene, propylene, C4+, N₂O) are also separated in the separator 17 at the cold end of the exchanger, either in solid form, or in liquid form.

The purified air 20 is divided into two portions 23, 25. The portion 23 is sent to the middle of the single distillation column 27 where it is separated to form nitrogen-enriched gas 47 at the top of the column and an oxygen-enriched liquid 43 at the bottom of the column 27.

The portion 15 of the air is condensed at least partially in a heat exchanger 59 by heat exchange with a flow of fluid 57 that is cooled by means of a heat pump 21 using the magnetocaloric effect.

A cooling fluid 53, typically ambient air or cooling water is sent to the heat pump 21 using the magnetocaloric effect. Reheated water 55 leaves the heat pump 21.

The column comprises a bottom reboiler 29 and an overhead condenser 31. The reboiler is heated by means of a fluid circuit 41 in connection with a heat pump 33 using the magnetocaloric effect. This heat pump 33 using the magnetocaloric effect also serves to cool a fluid 37 which cools the overhead condenser 31. The fluids 37 and 41 may be identical or different. An oxygen-enriched liquid 43 is drawn off at the bottom of the column 19 and a nitrogen-enriched gas drawn off via a pipe 47 is reheated in the exchanger 3 and is not used to regenerate a purification unit since there is none. An oxygen-enriched gas 45 is drawn off at the bottom of the column 27, is reheated in the exchanger 3 and is compressed by a compressor 49.

FIG. 1a illustrates a variant of FIG. 1 in which the heat exchanger 3 is constituted by two exchange bodies 3 a, 3 b. Each of the bodies 3 a, 3 b is a plate-fin exchanger as described above but other technologies may be envisaged.

Unlike FIG. 1, throughout the duration of the distillation, the air 1 is divided into two flows, one of which is sent to the body 3 a and the other to the body 3 b. Each impurity on a surface of the passages. The cooling passages of each body 3 a,3 b are at least partially covered by a coating and/or physically treated and/or chemically treated, the coating and/or the treatment serving to limit, or even prevent, the formation and/or the adhesion of the solidified impurity on a surface of the passages. It can be envisaged to use more than two bodies.

The solidified carbon dioxide and/or the (solidified) water is collected for both bodies and sent to a single container 17. The use of several containers can obviously be envisaged.

The purified air from both bodies is mixed to form the flow 20 and continues its treatment as for FIG. 1.

The gas 47 is reheated simultaneously in the two exchange bodies during the distillation, being divided into two upstream of the bodies 3 a, 3 b and remixed downstream of these bodies.

The gas 45 is reheated simultaneously in the two exchange bodies during the distillation, being divided into two upstream of the bodies 3 a, 3 b and remixed downstream of these bodies.

As each passage only receives one gas to be reheated or one gas to be cooled and the flows are not reversed during the distillation, the number of passages dedicated to cooling the air is not identical to the number of passages intended to reheat the gas 47 for a given body.

Illustrated schematically in FIG. 1b is an exchange body corresponding to a body 3,3 a,3 b of one of the other figures, where it is possible to observe that the number of passages dedicated to the cooling of the gas mixture, here air, is not identical to the number of passages dedicated to the reheating of a first gas, here nitrogen NR corresponding to the nitrogen 47 or to the number dedicated to the reheating of a second gas, here gaseous oxygen OG.

In the variant from FIG. 2 and those from FIGS. 3 to 9, the heat exchanger 3 is a monoblock heat exchanger that cools all the air intended for the distillation throughout the period where the distillation takes place. It also reheats all the gas originating from distillation throughout the period where the distillation takes place. The extraction of the water on the one hand then the remainder of the water and the CO₂ on the other hand are also carried out at two different locations in the exchange line 3. However, a large portion of the water is removed in solid form (around 97% of the water present in the air 1 on arriving in the exchanger 3) at a temperature close to −25° C., therefore in solid form. The air 5 is drawn off at this location by separating the air and the ice 5B in a phase separator 2 then the dried air 5A is reinjected in order to finish the cooling thereof. The air sent to the separator 2 has already been cooled upstream by a first refrigeration make-up at 0° C.

Thus air 7 drawn off at an intermediate level of the exchange line 3 is cooled firstly by means of a first heat pump 4 and then is removed from the line in order to remove the ice. The cooled air 5A sent back to the exchange line 3 is again cooled by the second cooler 6.

For the case of FIG. 3, the air is firstly cooled in the exchange line 3, it is taken out of the exchange line and water in liquid form is removed in the separator 2, next the purified air 5A is cooled in the exchange line, then by a first cooler 4, then in the exchange line 3, next the air is purified to remove the water in the separator 8, it is cooled in the exchange line, then with a second cooler 6 and the air is cooled up to the cold end.

It should be noted that in all cases, there may be only a single refrigeration make-up at the exchange line 3, or even none at all, if one is ready to sacrifice energy, the necessary refrigeration make-up then being introduced at the cold end of the exchanger, where it costs the most.

In the variant from FIG. 4, the order of the steps of separation of water and CO₂ and of refrigeration make-ups is as for FIG. 1. By drawing off liquid water in a first separation at around 0° C., this liquid water can be used to cool the machines, for example the compressor C.

But in this example, the solids or liquids 19 (the remainder of the water, the CO₂ and other secondary impurities) collected in the separator 17 are sent to the exchange line 3 in order to provide refrigeration thereto. This makes it possible to recover a portion of the latent heat, and therefore to reduce, or even simplify the necessary refrigeration make-ups.

In order not to complicate the exchange line, they may be injected into at least one dry and cold fluid, for example originating from the cryogenic separation, for example the nitrogen 47 in order to form a mixed flow 61. In this case, it may be prudent to treat at least certain portions of the nitrogen reheating passages in order to limit, or even prevent, the deposition of these solids.

FIG. 5 illustrates the case where an endless screw system 17A is used in order to extract the impurities in the form of ice or a mixture of ice/liquid in order to reinject them directly into the products. This replaces the phase separator 17 from the other figures.

Other means may be envisaged for removing the solid impurity, which may be released to the atmosphere. The heat exchanger may cool the gas containing at least one impurity periodically and the impurity may be melted, for example while the heat exchanger is not operating.

The solid could also be evacuated by consenting to lose a portion of the gas mixture that then transports the solid, with a pneumatic style transportation.

FIG. 6 is a variant where all of the water and CO₂ present in the air at the hot end are removed at the cold end of the exchange line. The refrigeration make-up may be provided to compensate for the condensation and the solidification of the impurities, and also the cooling thereof throughout the exchange line with a multitude of heat pumps, here n heat pumps PAC1, PACn supplied by cooling flows 9, 13. The refrigeration make-up may also be made with a sliding cold temperature. It may also be limited to 1 or 2 refrigeration make-ups.

In FIG. 7, a portion of the impurities is removed by a conventional adsorption separation system A. This portion may constitute between 20% and 95% of an impurity or of the impurities present. The purification may be made by means other than adsorption. Next, the fluid purified of at least one impurity enters the exchange line where the removal of the remainder of the impurities by solidification/liquefaction compensate for the latent heat of liquefaction and condensation of the impurities. It is possible to reinject the impurities into the products, as seen for FIGS. 4 and 5. In the case where the majority of the impurities to be removed are removed upstream in a conventional system A, the refrigeration make-up may be made solely at the cold end of the exchanger 3.

In the variant from FIG. 8, another separation system E is used at the outlet of the fan C in order to remove a portion of the impurities of the flow, for example in the form of a drying wheel. Next, the fluid 1 still loaded with impurities enters the exchange line 3. The impurities are removed at two levels and heat pumps compensate for the latent heat of condensation and liquefaction of the impurities. The frozen water/solid CO₂ and solid/liquid secondary impurities are recovered at the cold end without re-injecting them into the products.

In the case from FIG. 9, for a gas mixture 1 loaded with water in gaseous form, the water in liquid and/or solid form flows counter-current to the flow of gas 1, the stream of air 1 entering through the bottom of the exchanger 3, contrary to the conventional operation (it is also possible to imagine an inverted U-shaped exchanger configuration, with hot end and cold end at the bottom, and an intermediate point at the top).

Indeed, by solidifying and/or liquefying, the water becomes heavier and falls counter-current to the gas, which is cooled. It emerges in liquid form at the hot end of the exchanger 3.

This variant does not use phase separators but generally needs supplies of refrigeration at the exchange line.

Conversely, the cold flows 45, 47 enter through the top of the exchange line and exit through the bottom.

For greater clarity, the figure is drawn as if the water and/or the ice 19 descended through a passage other than the passage through which they entered, present in the air.

In fact, the water and/or the ice 19 will exit through the same passage through which they entered.

In the variant from FIG. 10, for a mixture containing water, the exchange line 3 from FIG. 9 is divided into two (exchange line 3 and 3A) in order to draw off the water between the two at an intermediate temperature. Thus the air cooled in the line 3 with a supply of refrigeration from the cooler 4 is separated in the phase separator in order to remove a portion of the water 5B. The remainder of the water and/or of the ice falls toward the bottom of the lines 3 and 3A. The at least partially purified air is cooled in the exchange line 3A with a supply of refrigeration from the cooler 6 and is then cooled in the line 3A again. The phase separator 17 is not present in this particular case.

For the case where the exchange line is divided into two exchange bodies in series, the two lines 3,3A may be constructed with the same technology or different technologies (plate-fin exchanger, coil exchanger, shell and tube exchanger). Similarly, if the exchange lines are of the same technology, they do not necessarily have the same construction and may differ by the dimensions of the passages, the number of passages, the type of coating and/or treatment used to limit the deposition of solids, the type of fins use, the material out of which they are constructed, etc.

These eleven examples all relate to the separation of air by distillation in a single column. The invention may be applied to the cryogenic separation of air by any known system of columns, other than that described and using any known means of producing refrigeration, other than those described.

The air separation apparatus may for example be a double air separation column producing at least one gaseous product and/or at least one liquid product.

The invention may also be applied to the purification and cooling of other gas mixtures having at least one impurity capable of solidifying during the cooling. An impurity is a component that represents no more than 10 mol % or 5 mol % or even 1 mol %, or even 0.1 mol %, or even 0.01 mol % of the gas mixture.

It is applied in particular to other gas mixtures, for example to mixtures of carbon dioxide containing for example at least 30% carbon dioxide and water. In this case, the passages of the exchange line are treated in order to limit, or even prevent, the deposition of the water and the pressure and the temperature are chosen in order to avoid the deposition of CO₂. A mixture of this type may be separated in a process from FIGS. 1 to 10 by modifying the operating temperatures.

For all the figures, the supply of refrigeration, if there is any, may be carried out with any known and suitable means (for example, magnetocaloric cooler, compression-expansion conventional refrigerating unit, turbine).

In the figures, a portion of the gas mixture leaves the exchange line in order to be cooled in the element supplying refrigeration while the remainder of the gas mixture continues its cooling in the exchange line. The portion cooled by the supply of refrigeration is then mixed with the remainder of the mixture which has not left the exchange line.

It is also possible to take the entire gas mixture out of the exchange line in order to send it to the element supplying refrigeration and send back the cooled mixture to the exchange line.

In the examples from the figures, it is seen that a portion or several portions 7, 11 of the air leave the exchanger 3 in order to be cooled and sent back (flows 7A, 11A) to the exchanger. It is also possible in all or some of the cases to use a heat transfer fluid in a closed circuit which transfers heat from the heat exchanger 3 to the cooling means 4,6 and returns to the heat exchanger in order to supply refrigeration thereto.

In all cases, it can be envisaged to provide a final purification downstream of the exchanger and, where appropriate, downstream of the phase separator or endless screw, in order to eliminate the remaining impurities in the flow of mixture 20.

In all cases, the heat exchanger 3 may comprise passages, at least one section of which has a treatment and/or a coating and/or a geometry and/or a type of fin, in the case of a plate-fin exchanger, that differs from that of another section that has to operate at a lower temperature range.

For example, the passage section of the exchanger that is at a temperature between 20° C. and 0° C. will be treated in one way or will have a coating of one type and the passage section that is at a temperature between 0° C. and −60° C. will be treated in another way. The treatment or coating may be chosen in order to adapt to the type of physical phenomenon change (gas→liquid, gas→solid, liquid→solid), or else the type of impurities in question (for example water/carbon dioxide).

The heat exchanger 3 may comprise passages, at least one section of which has a treatment and/or a coating and/or a geometry and/or a type of fin, in the case of a plate-fin exchanger, that differs from that of another section that is found downstream of an intermediate point for drawing off solidified impurities.

The heat exchanger may be constituted by at least two heat exchangers made of different materials, for example one brazed aluminum exchanger and one brazed copper exchanger.

For all the examples, during substantially the entire time that the separation by distillation is carried out, the gas mixture is cooled in each exchange body having cooling passages designed to reduce the adhesion of the solidified impurity on a surface of the passages. 

1-16. (canceled)
 17. A process for cooling, purifying and separating a gas mixture (1) containing at least one impurity of a gas mixture, the process comprising the steps of: cooling the gas mixture containing at least one impurity to a temperature below or equal to that at which the at least one impurity solidifies in a heat exchanger, the heat exchanger comprising at least one exchange body having cooling passages configured to reduce the adhesion of the solidified impurity on a surface of the passages; collecting at least one portion of the solidified impurity leaving the cooling passages of the heat exchanger and/or at an intermediate level of the heat exchanger; withdrawing the gas mixture from the heat exchanger; sending the gas mixture to a system of columns under conditions effective for separating the gas mixture by cryogenic distillation to produce at least a first fluid and a second fluid, wherein each of the first fluid and the second fluid are enriched in one component of the gas mixture, wherein the cooling passages are at least partially covered by a coating and/or physically treated and/or chemically treated, the coating and/or the treatment serving to at least limit the formation and/or the adhesion of the solidified impurity on a surface of the cooling passages, and wherein: i) during substantially the entire time that the separation by distillation is carried out, the gas mixture is cooled in each exchange body having cooling passages configured to reduce the adhesion of the solidified impurity on a surface of the passages, and/or ii) the first fluid and the second fluid are reheated in each exchange body having cooling passages designed to reduce the adhesion of the solidified impurity on a surface of the passages.
 18. The process as claimed in claim 17, wherein the gas mixture is purified to remove at least one fraction of the at least one impurity upstream of the heat exchanger, said one fraction representing between 20% and 95% of the impurity contained in the gas mixture upstream of the process.
 19. The process as claimed in claim 17, wherein at least one portion of the solidified impurity is collected downstream of the heat exchanger, by means of a phase separator and/or an endless screw.
 20. The process as claimed in claim 17, wherein the cooled gas mixture is treated downstream of the heat exchanger and/or at at least one intermediate level of the heat exchanger in order to eliminate the impurity in gaseous and/or liquid and/or solid form.
 21. The process as claimed in claim 20, wherein the gas mixture is cooled in the heat exchanger firstly to a temperature below or equal to the liquefaction temperature of the at least one impurity but above its solidification temperature, at least one portion of the gas mixture is taken out of the exchanger in order to eliminate a portion of the impurity in liquid form and at least one portion of the gas mixture containing some impurity is sent back to the heat exchanger in order to cool the at least one portion of the gas mixture containing some impurity to the solidification temperature of said impurity.
 22. The process as claimed in claim 20, wherein at least 50% of the impurity present at the inlet of the heat exchanger, referred to as the hot end, is eliminated by collecting the impurity downstream of the heat exchanger after cooling up to the outlet of the exchanger at the cold end.
 23. The process as claimed in claim 22, wherein the hot end of the heat exchanger is placed at a higher level than that of the cold end.
 24. The process as claimed in claim 17, wherein the hot end of the heat exchanger is placed at a lower level than that of the cold end or than that of an intermediate level of the exchanger in the case of an inverted U-shaped exchanger and at least 50% of the impurity present in the gas mixture to be cooled is eliminated at the inlet of the heat exchanger, referred to as the hot end, by collecting the impurity in solid or liquid form at the hot end of the exchanger where the impurity drops by gravity after cooling in the heat exchanger.
 25. The process as claimed in claim 17, wherein the gas mixture is: air with the at least one impurity being selected from the group consisting of water, carbon dioxide, and combinations thereof, or a mixture of gases, having a main component selected from the group consisting of hydrogen, carbon monoxide, methane, and combinations thereof, with the at least one impurity being selected from the group consisting of water, carbon dioxide, and combinations thereof, or a mixture of gases, having the main component of carbon dioxide and at least a second component selected from the group consisting of hydrogen, carbon monoxide, methane, oxygen, nitrogen, argon, and combinations thereof, with the at least one impurity being water.
 26. The process as claimed in claim 17, further comprising the step of treating at least one surface of the cooling passages thereby producing a surface that is hydrophobic and/or superhydrophobic and/or that has hydrophobic and hydrophilic, and/or hygroscopic zones, in order to at least limit the formation and/or the adhesion of solidified impurities.
 27. The process as claimed in claim 17, wherein the heat exchanger comprises at least one passage for reheating a fluid, the at least one reheating passage not having been treated or coated in order to at least limit the formation and/or the adhesion of solidified impurities.
 28. The process as claimed in claim 17, wherein at least one portion of the solidified impurity leaving the cooling passages of the heat exchanger is sent back to the heat exchanger to be reheated.
 29. The process as claimed in claim 17, wherein at least one portion of the solidified impurity leaving the cooling passages of the heat exchanger and/or at an intermediate level of the heat exchanger is collected and the gas mixture finds itself liquefied or is liquefied by a subsequent step downstream of the exchanger and/or is separated at a subambient temperature downstream of the exchanger, optionally after elimination, downstream of the exchanger, of remaining impurities that would be solidified at this subambient temperature.
 30. The process as claimed in claim 17, wherein frigories are supplied to the gas mixture which is cooled at at least one intermediate point of the heat exchanger downstream or upstream of a point for drawing off at least one portion of the solidified or liquefied impurity at an intermediate level of the heat exchanger.
 31. The process as claimed in claim 17, wherein the gas mixture is at least partially liquefied within the heat exchanger and is withdrawn from the cold end of the heat exchanger.
 32. The process as claimed in claim 17, wherein the gas mixture is cooled downstream and
 33. An apparatus for cooling and purifying a gas mixture containing at least one impurity comprising a heat exchanger comprising at least one, or even two, exchange bodies, each having cooling passages designed to reduce the adhesion of the solidified impurity on at at least one portion of the surface of the passages and reheating passages, means for sending the gas mixture containing at least one impurity to be cooled in the cooling passages of the exchange body or bodies to a temperature below or equal to that at which the at least one impurity solidifies and means for drawing off the, optionally at least partially liquefied, gas mixture from the exchange body or bodies, preferably at the cold end, means for sending a gas to be reheated in the reheating passages and means for collecting at least one portion of the solidified impurity leaving the cooling passages of the heat exchanger and/or at an intermediate level of the heat exchanger and means for taking the gas mixture of the at least one impurity out of the heat exchanger, wherein the cooling passages of each exchange body are at least partially covered by a coating and/or physically treated and/or chemically treated, the coating and/or the treatment serving to limit, or even prevent, the formation and/or the adhesion of the solidified impurity on a surface of the passages and in that reheating passages are connected to means for transporting a first gas to be reheated and other reheating passages are connected to means for transporting a second gas to be reheated.
 34. The apparatus as claimed in claim 33, wherein the number of cooling passages is not equal to the number of reheating passages connected to the means for transporting the first gas and the number of cooling passages is not equal to the number of reheating passages connected to the means for transporting the second gas. 